Liquid crystal displays (LCD) typically include a liquid crystal panel which is backlit with a white light source. White light generated by the source passes through individual pixels of the liquid crystal panel and is color filtered. A user viewing the LCD sees such color-filtered light as the image generated by the LCD.
Known white light sources include cold cathode fluorescent lamps (CCFLs). Other white light sources include colored light emitting diodes (LEDs). Typically, such LED-based white light sources include clusters of three LEDs, one emitting blue light, and the other two emitting green and red light, respectively. In each cluster, the LEDs are positioned close to one another so that the light from each is mixed with the other LEDs of the cluster. The combined output of the red, blue, and green light output from each cluster thus appears white. Many such LED clusters are often provided to illuminate the entire liquid crystal panel.
LED-based white light sources are advantageous in that they output light over a broader range of colors and have better color saturation than many CCFLs.
In order for white light to be emitted from the LED clusters, the light intensity associated with each LED is typically maintained at a particular value. Over time, however, each LED tends to emit less light, and the rate of such decaying light intensity varies for each LED. As a result, the white light source may appear to have a colored hue, either over the entire display or in localized portions, instead of being white. Changes in temperature can also create such a colored hue by affecting the intensity of light output by the LEDs.
In order to maintain the desired light intensity output from each LED, i.e., maintain a desired “color balance,” a feedback system may be provided to compensate for the above-noted color variations. Namely, detectors may be provided adjacent the white light source in order to detect the overall intensity of red, blue, and green light emitted by the source. If an excess amount of blue light is detected, for example, a control circuit may adjust the current supplied to the red, blue, and green LEDs of the LCD so that the overall intensity of red, blue, and green light output from the source is at a desired level.
Since the feedback circuit monitors the light intensity of the white light source as a whole, it cannot ensure that white light is generated by individual clusters of LEDs. As a result, portions of the white light source may still not have a desired color balance, even when the above-noted feedback circuit is employed.
In addition, the current-voltage (I-V) curve associated with each LED is non-linear, such that small changes in voltage result in disproportionate changes in current. Accordingly, the current flowing through each LED (and thus the brightness or intensity associated with each LED) is typically not controlled by adjusting the voltage across the LED. Rather, current pulses are applied to each LED instead, whereby the width of each pulse is either widened or shortened in order to increase or decrease the total amount of current supplied to each LED. Such pulse width modulated (PWM) current, however, often does not supply a sufficient amount of current for the LEDs to generate a maximum light intensity. The maximum light intensity can be achieved, however, with known current driving integrated circuits (ICs), but such ICs typically supply the desired amount of current to a limited number of LEDs. Accordingly, often many such current driving ICs are necessary in order to provide the desired amount of current to each LED, thereby increasing the cost of LCDs including LED-based white light sources.